sodium silicate any one of several compounds containing sodium oxide, Na ₂ O, and silica , Si ₂ O, or a mixture of sodium silicates. Sodium ortho silicate is Na ₄ SiO ₄ (or 2Na ₂ O·SiO ₂ ); sodium meta silicate is Na ₂ SiO ₃ (or Na ₂ O·SiO ₂ ); sodium di silicate is Na ₂ Si ₂ O ₅ (or Na ₂ O·2SiO ₂ ); sodium tetra silicate is Na ₂ Si ₄ O ₉ (or Na ₂ O·4SiO ₂ ). All these compounds are transparent, glassy or crystalline solids that have high melting points (above 800°C) and are water soluble. They are produced chiefly by fusing sand and sodium carbonate in various proportions. The product is commonly known as water glass . The greatest single use of sodium silicates is as a raw material for making silica gel .

rock-forming silicate minerals About one-third of all known mineral species are silicates. Many of them are rare, but a few families—the olivines, pyroxenes, amphiboles, micas, clay minerals, feldspars, quartz, and garnets — make up some 95 per cent of the Earth's crust. According to on one particular model, the mineralogy of the upper mantle is probably 37–51 per cent olivine, 26–34 per cent orthopyroxene, 12–17 per cent clinopyroxene, and 10–14 per cent garnet. Since the mantle extends down to 2900 km it is clear that the most abundant mineral on Earth is olivine or its equivalent.

Silicates crystallize under widely differing physico-chemical conditions ranging from those at the base of the mantle to low-temperature, low-pressure, watery environments in the pores of sandstones. They may have highly variable complex compositions; on the other hand, many minerals are made from the same restricted range of compositions but under different conditions.

Silicate minerals are classified primarily on the basis of their crystalline structure. The essential basic unit is the silicon–oxygen tetrahedron with one silicon atom surrounded by and strongly bonded to four oxygen atoms located at the apices of the tetrahedron to give an (SiO₄)⁴⁻ anionic unit. The SiO₄ units may be present as isolated tetrahedra linked by cations or they may progressively ‘polymerize’ by sharing one or more oxygen atoms with adjacent tetrahedra so that ultimately all oxygen atoms are shared to give a framework structure (Fig. 1).

Olivines

‘Olivine’ is the general name given to a group of magnesium–iron silicates with a composition that is mainly (Mg,Fe)₂Sio₄ with minor amounts of nickel, manganese, and calcium. It is a continuous solid-solution series from forsterite (Mg₂SiO₄) to fayalite (Fe₂SiO₄). There are names for intermediate members of the series, but it is now more usual to quote compositions in terms of the percentage of forsterite present; e.g. Fo₂₀.

The melting and crystallization relations of dry olivine under atmospheric conditions are shown in Fig. 2a. The upper curve is the liquidus, the lower one the solidus; above the liquidus olivine is liquid, below the solidus it is solid, and between the two curves olivine and liquid coexist. Consider a liquid of Fo₅₀ composition at 1900 °C (point A on the diagram), and let it cool. The cooling liquid follows a vertical line until it meets the liquidus (at point B) at a temperature of about 1675 °C. It then begins to crystallize, but the composition of the first crystal to form is not Fo₅₀ but Fo₈₀. This figure is derived by drawing a horizontal line from B to the solidus and then dropping a perpendicular to the base at C. As the temperature falls, the composition of the crystal is always richer in magnesium than the liquid in which it is growing. At 1410 °C all the liquid has crystallized, and if the cooling has been slow the result is a homogeneous olivine of Fo₅₀ composition. If, however, at some stage the olivine crystals are removed, the liquid will have been depleted in magnesium. This process of fractional crystallization exemplified here by the olivines is important in the derivation of a variety of magmas from a single homogeneous parent.

Olivine has orthorhombic symmetry, a hardness of 6.5 and no marked cleavage. Its density varies with composition, and its colour becomes a deeper green as the iron content increases; it may then be the gemstone peridot. Olivine is prone to alteration: mainly to serpentine, sometimes to chlorite.

Olivine is common as phenocrysts in basalts, as a major component in some dolerites and gabbros, and as the dominant mineral of ultramafic rocks such as peridotites and dunites. Fayalite occurs in acid lavas.

Pyroxenes

There are some 21 species in the pyroxene group, many of them rare. The pyroxenes are anhydrous single-chain silicates with the chains parallel to the z axis of the crystals. The chains are linked by cations, predominantly calcium, iron, and magnesium. The general formula is X₁₋p Y₁₊p Z₂ O₆, where p can have values between 0 and 1; X can be calcium or sodium, Y can be manganese, iron (Fe²⁺ Or Fe³⁺), magnesium, aluminium, chromium, or titanium; and Z can be silicon or aluminium. A wide range of substitutions is possible.

Figure 2b shows the range of compositions of the common pyroxenes. Calcium-rich species in the wollastonite area are not pyroxenes but pyroxenoids. There are three principal series: (1) Mg₂Si₂O₆–Fe₂Si₂O₆, a complete solid-solution series from enstatite to ferrosilite. Members with less than 5 per cent Ca₂Si₂O₆ are orthorhombic; all other pyroxenes are monoclinic. Enstatite occurs in plutonic basic and ultrabasic rocks such as peridotite and norite, and also in some high-grade metamorphic rocks. (2) Diopside–hedenbergite, a complete solid-solution series formed mainly by the metamorphism of siliceous dolomites and impure iron-rich carbonates. (3) Augite, with a more complex composition including aluminium, is a very common phenocryst in basalts and is the commonest pyroxene.

Pyroxenes have two good vertical cleavages intersecting at 87°, variable, mainly dark, colours, a hardness of about 6, and are prone to alteration to hornblende, chlorite, and clay minerals.

Some members of the pyroxene family are used as geothermometers and geobarometers.

Amphiboles

Some 60 species make up the amphibole family of double-chain silicates. The chains are bonded by chains of cations, all chains being parallel to the vertical axis. In contrast to the pyroxenes they all have hydroxyl (OH) and several have fluorine or chlorine in their structure; there is a wide and complex variation in chemical composition and sometimes extensive solid solution. The amphiboles are stable over a wide range of physical conditions in both igneous and metamorphic rocks.

The general composition of the amphiboles can be written as W₀₋₁X₂Y₅Z₈O₂₂(OH, F, Cl)₂, the W cations being Ca, Na, or K; the X cations Ca, Mg, Fe, or Mn; the Y cations Fe, Ti, or Al; and the Z cations being Al or Si. With a few exceptions amphiboles are monoclinic with two good prismatic vertical cleavages intersecting at angles of 56° and 124° (another feature that distinguishes them from pyroxenes). They are usually prismatic or fibrous, elongated parallel to the vertical axis, with a hardness of 5.5–6, a specific gravity of 2.9–3.5, and a wide range of colours varying with composition.

Hornblende covers three series with a range of compositions in which sodium and aluminium are important. Common hornblende is the most widespread of the amphiboles, with a dark green to black colour. It is present in most major groups of plutonic igneous rocks, especially the calc-alkali intermediate ones. It is common in rocks formed by the medium to high-grade metamorphism of impure limestones and basic igneous rocks to give amphibolites.

The tremolite–actinolite series is non-aluminous and varies in composition from Ca₂Mg₅ to Ca₂(MgFe)₅. These minerals are white, becoming green with increasing iron. They are usually fibrous; there is a densely felted variety, nephrite, formed by low- to medium-grade metamorphism of siliceous dolomites with additions of iron.

Glaucophane–riebeckite are sodium-bearing, the main substitutions being between magnesium, aluminium, and iron (Fe²⁺ and Fe³⁺). Their colour varies from pale blue to lavender blue to blue-black. One of the varieties is the dangerous blue asbestos, crocidolite. Glaucophane and riebeckite are found in low-grade schists of basic igneous parentage; crocidolite occurs in medium-grade metamorphosed ironstones. Riebeckite is also found in granites and syenites.

Micas

Of the 30 or so micas, only biotite, muscovite, and phlogopite are common and widespread. The micas are sheet silicates in which oxygens at three corners of the tetrahedron are shared with neighbouring tetrahedra, leaving one free oxygen. Each sheet of tetrahedra has hexagonal symmetry; two such sheets are linked by cations to give three-layer units, which are usually linked by potassium atoms with weak bonding. This results in perfect cleavage parallel to the basal plane of the minerals that gives flexible and elastic sheets.

The general composition of the group can be written as W_0–1 Y_2–3(Z₄O₁₀) (OH,F)₂ the main atoms in the W position are K, Ca, Na, those in Y are Al, Fe, Mg, Li, and those in Z are Al or Si (usually Al Si). All members contain hydroxyl (OH); some have fluorine. They crystallize in the monoclinic system as hexagonal books with perfect basal cleavage and a platy habit; hardness varies from 2 to 4 and specific gravity from 2.70 to 3.30 according to composition, as does colour from colourless to violet, bright green, silvery, and dark brown to black.

Biotite, K(Mg Fe)₃(Al Si₃O₁₀) (OH)₂, is the most common mica. It occurs in most igneous rock groups but is mainly in intermediate plutonics and in pelitic schists and gneisses; it is an index mineral in low-grade schists of regional metamorphic origin.

Phlogopite, K Mg₃ (Al Si₃O₁₀) (OH)₂ occurs in kimberlites and metamorphosed siliceous dolomitic limestones.

Muscovite, K Al₂(Al Si₃O₁₀) (OH)₂, is an important constituent of pelitic schists and gneisses, of some granites and granite pegmatites, and of greisen. Sericite is fine-grained muscovite formed by alteration of such minerals as feldspar, cordierite, and sillimanite.

Lepidolite, K(Li Al)₃(Al Si₃O₁₀) (OH, F)₂ is found in pegmatites and is a source of lithium.

Glauconite is close in composition to muscovite but has some iron, magnesium, sodium, and calcium. It is forming at present in marine environments where sediment supply is limited, and so is authigenic. Because it contains potassium it can be dated by radiometric techniques and can thus give the age of the sedimentary rock directly.

The main use of micas, particularly muscovite and phlogopite, is as sheets for electrical and thermal insulation, and as windows in stoves and furnaces; the variety vermiculite, when treated, is used for insulation blocks and for horticultural purposes.

Feldspars

Feldspars are the most important group of minerals, making up 60 per cent of the rocks of the Earth's crust. They are framework aluminium silicates of potassium, sodium, calcium, and (minor) barium with all the tetrahedral oxygen atoms shared; this would theoretically make the composition SiO₂, but in most feldspars one or two in four silicon atoms are replaced by aluminium. This substitution results in a charge imbalance that is offset by the addition of the cations sodium, potassium, or calcium, which fit into spaces in the structure. The compositions can be represented on a triangular diagram (Fig. 3a), in which the apices represent the three end-members orthoclase, albite, and anorthite. Nearly all feldspars so far analysed plot within the shaded area, and it can be seen that they form two series, both of which are solid-solution series: orthoclase to albite, the alkali feldspars, and albite to anorthite, the calc-alkali or plagioclase feldspars. There are considerable structural variations in the feldspars according to their temperature of formation, which influences the ‘ordering’ of the aluminium and silicon atoms and the composition.

Alkali feldspars. At temperatures above about 660 °C complete solid solution occurs between the two alkali feldspar end-members sanidine and high albite, but below this there is a break and two feldspars of different composition occur together. The result is an intergrowth of the two, usually with thin lamellae of albite in a host of orthoclase or microcline; this intergrowth is called perthite if visible to the naked eye or microperthite if visible only under the microscope. This differentiation within one crystal is called exsolution and takes place in the solid state.

Potash feldspars. The potash feldspars are polymorphic: sanidine is the monoclinic high-temperature form, microcline the triclinic low-temperature form, and monoclinic orthoclase an intermediate form. Sanidine is found only in quickly cooled lavas such as trachytes; microcline occurs in slowly cooled granites. The potash feldspars are usually pale coloured: glassy clear, white, pink or, rarely, bright green (as the variety amazonite); their hardness is about 6, their specific gravity 2.56–2.71. There are two good cleavages parallel to (001) and (010), at right angles in orthoclase and sanidine and at nearly 90° in microcline. Interpenetrant twinning on the carlsbad law is characteristic of sanidine and orthoclase (Fig. 3d).

Plagioclase. There is complete solid solution among the members of the plagioclase series at high temperatures, but complicated structures and a somewhat incomplete series exist at lower temperatures. This important series is arbitrarily divided into six distinct minerals easily recognized under the microscope but not in hand specimen. The boundaries and names are given in Fig. 3a; compositions are given in terms of the proportion of anorthite and are written, e.g., An₇₅. The melting–crystallization relations of the plagioclases are shown in Fig. 3b. The similarity with those of the olivines is obvious; under atmospheric conditions anorthite melts or crystallizes at 1540 °C, albite at about 1100 °C; a plagioclase of An₅₀ composition will begin to melt at 1280 °C but will not be completely molten until 1440 °C. By analogy with the olivines it will be seen that the first crystals to form will be rich in calcium, thus enriching the liquid in sodium. Zoning often results in crystals having centres rich in calcium and rims rich in sodium. Plagioclases are triclinic, white or pale green, with hardness 6–6.5 and specific gravity 2.6–2.75. They twin on a variety of laws, the most common being the albite law in which the twins are multiple, parallel to (010), and visible as bands in hand specimen; pericline twins are also multiple (Fig. 3c).

Plagioclases are common in most kinds of igneous rocks. They range from bytownite–labradorite in basalts and gabbros, andesine–oligoclase in diorites and andesites to albite in granites and rhyolites. They are major constituents of metamorphic rocks such as pelitic schists and gneisses and amphibolites; the composition becomes more calcium-rich with increasing temperature of formation. Pure anorthite is rare in igneous rocks but is found in calc-silicate rocks formed by the metamorphism of impure siliceous limestones and marls.

Feldspathoids

The feldspathoids are a group of comparatively rare anhydrous aluminous framework silicates of sodium and potassium; there are large cavities in the structure, which can accommodate K, Na, Co²⁻₃, and SO²⁻₄ions. The feldspathoids are similar to the feldspars but contain less silica; they are unstable in the presence of quartz and are characteristic of undersaturated (silica-poor) rocks.

The three commonest feldspathoids are leucite, nepheline, and sodalite.

Leucite (K Al Si₂O₆) is tetragonal (but pseudocubic) at temperatures below 625 °C; cubic, icositetrahedral, above 625 °C. It is unique among silicates in being less dense than the liquid from which it crystallizes. It occurs in lavas and hypabyssal intrusions but not in plutonic rocks. A well-known locality is that of Mount Vesuvius.

Nepheline (Na Al Si O₄) is hexagonal. It occurs in lavas and some plutonic rocks, such as syenites.

Sodalite (Na₈Al₆Si₆O₂₄) (Cl)₂ is cubic, dodecahedral. It occurs in nepheline-syenites. Lazurite, the deep blue variety of sodalite with sulphur replacing some of the chlorine, is dominant in lapis-lazuli.

In general the feldspathoids are pale-coloured, with a hardness of about 6 and specific gravity about 2.5.

Garnets

Garnets are a complex group of island silicates characteristic of metamorphic rocks, rare in igneous rocks such as rhyolites and granites and also in kimberlites, serpentinites, and eclogites, and as detrital grains in sediments. The general formula is X₃Y₂ (SiO₄)₃, in which X can be magnesium, iron, manganese, or calcium and Y can be aluminium, iron, or chromium. The garnets are assigned to two main groups: pyralspites, consisting of pyrope, Mg₃Al₂ (SiO₄)₃, almandine, Fe₃Al₂(SiO₄)₃ and spessartine, Mn₃ Al₂(SiO₄)₃; and ugrandites, consisting of grossular Ca₃Al₂ (SiO₄)₃, andradite, Ca₃Fe₂(SiO₄)₃ and uvarovite, Ca₃Cr₂(SiO₄)₃. There is much compositional variation within the groups but little between them. They are stable over a wide range of temperature and pressure. For example, in the Barrovian metamorphic complex of the Scottish Highlands they occur in rocks that crystallized over the range 350–650 °C and up to 12 kbar pressure. The garnets crystallize in the cubic system, with rhombdodecahedra and icositetrahedra as the common forms; they have poor cleavage, a hardness about 7, specific gravity from 3.4 to 4.6, and highly variable colour: dark red to almost black, colourless, brown, green, greenish yellow, rose red, yellow, orange, red-brown, or emerald green, according to the composition.

Almandine is the most common garnet. It occurs in middle- and high-grade schists and gneisses derived from pelitic rocks; it is the index mineral of one of the Barrovian zones and can also be found in contact-metamorphic aureoles. Grossular results from both regional and thermal metamorphism of impure limestones; spessartite and andradite are mainly found in metasomatic skarn deposits; pyrope occurs in kimberlites and eclogites; uvarovite is rare and is found in serpentinites, usually associated with chromite, and rarely in limestone skarns.

The main uses of garnets are for abrasives and grinding materials; they also provide semi-precious gemstones such as rhodolite, demantoid, and grossular.

Aluminium silicates

The aluminium silicate minerals are an important group of island silicates occurring almost exclusively in metamorphic rocks derived from pelitic protoliths. Three of them, andalusite, kyanite, and sillimanite, form one of the best-known examples of polymorphism with a composition Al₂Si O₅ or Al₂O₃. SiO₂. Figure 2c shows the possible stability relations. Sillimanite is the high-temperature form and cannot exist stably below 500 °C; kyanite is stable at high pressure, and andalusite at low pressure. At temperatures above 1300°C sillimanite converts to mullite plus silica liquid. (Mullite is not a polymorph because it has a different chemistry, Al₆O₅ (SiO₄)₂ or 3 Al₂O₃ 2 Si O₂.)

Andalusite is orthorhombic, crystallizing as elongate square prisms which at low temperature have a conspicuous cross-shaped arrangement of dark inclusions giving the variety chiastolite. There are two good vertical cleavages almost at right angles. The hardness is 7.5. The colour is usually grey or white, occasionally violet. Andalusite is mainly a contact-metamorphic mineral, but it also occurs in regional schists formed at low pressure.

Kyanite is triclinic with three cleavages and variable hardness (4–7 on a single face). In colour it ranges from pale green to blue to colourless. It forms good porphyroblasts, often bent, in schists and gneisses and is an index mineral in the Barrovian zonal system.

Sillimanite usually grows as elongate orthorhombic prisms but often as felted laths when it is known as faserkiesel. It is grey- or greenish-white with a hardness of 6 to 7 and a single good vertical cleavage. It is the index mineral of the highest grade of regional metamorphism.

All three minerals alter to shimmer aggregates of sericite; all three are used in the preparation of refractories.

Staurolite has a similar structure to kyanite but has a high iron content and contains some magnesium. It is pseudo-orthorhombic, short or long columnar, with hardness 7.5. In colour it varies from yellowish to dark reddish brown. Staurolite is well known for its interpenetrant twins, the two parts being related at angles of 60° or 90°; the latter are known as ‘fairy crosses’. Staurolite is characteristic of medium- to high-grade regional metamorphism of argillaceous parents rich in iron.

Zeolites

The zeolites are a group of 40 or so hydrated aluminosilicates with a very open framework structure of (SiAl) O₄ tetrahedra in which sodium and calcium, more rarely potassium and barium, strontium, and water molecules (H₂O) fit into the interconnecting cavities and channels. The water can be driven off as a continuous and partially reversible process without damaging or disrupting the structure; the ‘cavity’ atoms are deposited in the holes. There are two main groups: (1) fibrous with chains of tetrahedra in the framework and represented by natrolite Na₂ (Al₂ Si₃ O₁₀).2H₂O with acicular habit; (2) equant or cagelike, such as chabazite Ca₂(Al₂Si₄O₁₂). 6H₂O.

Because of their open structure zeolites have a low specific gravity ranging from 1.9 to 2.4, a hardness of 3.5–5.5, and are usually colourless or white when pure. They are, in general, late-stage minerals deposited from solutions in vesicles or joints in lavas, but phillipsite is a neoformational mineral that is being formed at present in deep-sea sediments at the water–sediment interface at a temperature of about 4 °C. Zeolites can also be produced by low-grade metamorphism of volcanic glass or tuffaceous sediments.

Zeolites are used as molecular sieves to remove large ions or complexes from solution, i.e. as absorbents, dessicants, and purifiers; in catalytic cracking in the petroleum industry; as fillers in papers; and as cationic exchangers in water softening. Many zeolites are now made synthetically.

Quartz and the clay minerals are dealt with in separate entries.

R. Bradshaw

Bibliography

Deer, W. A.,, Howie, R. A.,, and and Zussman, J. (1966) An introduction to the rock-forming minerals. Longman, London.
Schumann, W. (1992) Rocks, minerals and gemstones. Harper Collins, London.

water‐glass Sodium silicate; used at one time to preserve eggs, by forming a layer of insoluble calcium silicate around the shell, and so sealing the pores.